1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system of a copying machine, a laser printer, a facsimile, a printer, or a multifunction peripheral of them.
2. Description of the Related Art
In the related art, in an image forming apparatus using an electrophotographic system, a toner image is formed by developing an electrostatic image, which is formed on an electrophotographic photoreceptor (photoreceptor) by an exposure device, with a developing device. Further, the toner image is transferred to a recording material, for example, by an intermediate transfer member or a recording material conveying member, and then the toner is fixed on the recording material.
In the image forming apparatus using an electrophotographic system, when a toner image is electrically transferred to a transfer target member (such as a recording material on the intermediate transfer member or the recording material conveying member) from a transfer portion, transfer residual toner of about 1 to 10% is generated. The transfer residual toner passes through the transfer portion and turns around the photoreceptor. A method of using a cleaning device using cleaning blades is typically provided as a method of recovering the transfer residual toner from the photoreceptor. Further, there is also a method of reusing toner recovered in a developing device by providing a charging-assistant member and performing cleaning simultaneously with development with the developing device (see U.S. Patent Application Publication No. 2008/0285993 A1).
It is possible to easily perform cleaning simultaneously with development by applying a direct voltage to the charging-assistant member to recharge the transfer residual toner. Therefore, it is desirable that the voltage that is applied to the charging-assistant member be a voltage of a discharge threshold or more.
Further, the timing of applying a voltage to the charging-assistant member needs to satisfy the following conditions A and B.
The condition A is to apply a voltage to the charging-assistant member until the position of the photoreceptor charged by applying a voltage to a charging member comes to the position of the charging-assistant member. With the charged photoreceptor at the position of the charging-assistant member, when the voltage is not applied to the charging-assistant member, the potential (0 V) of the charging-assistant member becomes larger than the potential of the charged photoreceptor. Thus, positively-charged toner sticking to the charging-assistant member is abnormally discharged from the charging-assistant member to the photoreceptor and contaminates the charging member through the photoreceptor, which causes lack of potential or an image defect due to fog.
The condition B is to apply a voltage to the charging member until the photoreceptor charged by the charging-assistant member comes to the position of the charging member. When the potential (for example, −400 V) of the photoreceptor charged by the charging-assistant member reaches (development potential of 0 V of) the developing portion, the potential difference (Vback) between the potential of the photoreceptor and the development potential at the development position becomes abnormal, such that carriers stick or fog of the toner is generated, thereby causing an image defect. The development potential is the potential of a direct-current component of a development voltage that is applied to a developer bearing member of the developing device.
For the reasons described above, it is necessary to apply a voltage to the charging-assistant member such that the conditions A and B are satisfied.
As the image forming apparatus having an electrophotographic system, there is a tandem type of an image forming apparatus including four image forming portions that form an image of four colors, for example, of yellow (Y), magenta (M), cyan (C), and black (K), respectively. In this case, the space for disposing a high-voltage power supply that is a voltage-applying unit or a transformer is downsized and the cost decreases as compared with when a high-voltage power supply or a transformer is individually provided, by sharing the voltage-applying unit which applies a voltage to the charging-assistant members of a plurality of image forming portions (see Japanese Patent Laid-Open No. 2002-148894).
However, when the voltage-applying unit which applies a voltage to the charging-assistant members in the plurality of image forming portions is shared, the timings of applying a voltage to the charging-assistant members in all the image forming portions become the same.
Therefore, for example, when the timing of applying a voltage to the charging-assistant member is set such that the conditions A and B are satisfied in the image forming portion at the uppermost-stream side, the following problems are generated. That is, in this case, it is necessary to greatly increase the time of applying a voltage to the charging member in order to satisfy the conditions A and B for the timing of applying a voltage in the image forming portion at the lowermost-stream side. That is, a voltage starts to be applied to the charging member of the image forming portion at the lowermost-stream side in accordance with the charging timing at the uppermost-stream side. As a result, in the image forming portion at the lowermost-stream side, cutting of the photoreceptor or fusion bonding (filming) of the toner is made worse, which causes a longitudinal line-shaped defect in the image and reduces the durability life of the photoreceptor.